(1) Field of the Invention
The present invention relates to optical fiber composite insulators and processes for producing the same.
(2) Related Art Statement
a) Power transmission lines and power substations require systems for rapidly detecting locations of any troubles occurring in the power transmission lines or the power substations due to lighting, etc. and for restoring the systems. Therefore, abnormal current or abnormal voltage detectors utilizing optical sensors having Faraday effects and Pockels effect have been used. In these detectors, it is necessary to insulate the voltage and current in the power transmission between the sensor attached to a power transmission line and the troubled location detector. For this purpose, optical fiber composite insulators in which optical fibers are placed are used to transmit optical signals only and maintain electrical insulation.
As such optical fiber composite insulators, it is a common practice that a slender through hole is provided in the insulator body thereof, an optical fiber is passed through this through hole, and the optical fiber is sealed in the through hole with an organic insulating material such as silicone rubber or an epoxy resin. However, there is a problem in that the organic insulating material is largely shrunk at low temperatures in the winter season so that the optical fiber is warped to increase loss in the light transmission. Further, there is another problem in that the organic insulating material does not go around the optical fiber if organic insulating material-pouring conditions are not properly kept during the production of the optical fiber composite insulator so that poorly adhered locations are likely to be formed to reduce insulating performance.
b) Moveover, Japanese Utility Model Registration Laid-open No. 64-31,620 proposed an optical fiber composite insulator in which a through hole is provided in a slender insulator body, an optical fiber is passed through the through hole, and an organic insulating material is filled in this through hole in the state that the organic insulating material is swelled up from end faces 2 of the insulator body. This is to absorb expansion of the organic insulating material with the swelled portions of the organic insulating material so that swelling of the interior organic insulating material itself out of the through hole and consequent breakage of the optical fiber may be prevented. Since the expanded amount of the organic insulating material is great at high temperatures, it is a common practice that the organic insulating material is largely swelled up to absorb the expansion as much as possible.
However, such optical fiber composite insulators also have a problem in that the light transmission loss increases particularly at low temperatures. Furthermore, the insulators have another problem in that adhesion forces decrease between the swelled-up portion made of the organic insulating material and the end face of the insulator body, when the insulator undergoes temperature changes over an extended time period.
c) Furthermore, NGK proposed an optical fiber-holding structure as an optical fiber composite insulator in Japanese Utility Model Registration Application No. 3-87,080 (filed on Sept. 27, 1992, not published) schematically shown in FIG. 1. This structure will be briefly explained.
A through hole 1a is provided in a central portion of an insulator body 1, and for example, two optical fibers 2 are passed through the through hole 1a. Optical fibers 2 are gas-tighly sealed inside the through hole 1a with an organic insulating material 3. In the embodiment of FIG. 1, the organic insulating material 3 is swelled up from an end face 1b of the insulator body 1 to form a swelled portion 4. This swelled portion 4 consists of three portions. That is, a frusto-conical portion 4a is concentrically formed around the through hole 1a, a columnar top portion 4c is formed on a central portion of the frusto-conical portion 4a, and a relatively thin extended portion 4b is formed at a skirt of an outer peripheral edge of the frusto-conical portion 4a. The optical fibers 2 are passed through the frusto-conical portion 4a and the columnar top portion 4c, and comes out from an end face 4d of the columnar top portion 4c.
A peripheral side face of the columnar top portion 4c is covered with a cylindrical pipe 22. Portions of the optical fibers 2 not sealed with the organic insulating material are passed through protective tubes 25. Parts of the optical fibers are exposed between end faces of the protective tubes 25a and the end face 4d of the columnar top portion 4c. A molding adhesive is poured and filled into the pipe 14, thereby forming a molding layer 24. The exposed portions 2a of the optical fibers and the near end faces of the protective tubes 7 are fixed and held inside the molding layer 24.
However, it is first discovered that such a holding structure has the following problems.
That is, since the optical fibers 2 and the tip portions of the protective tubes 25 are directly fixed inside the molding layer 24, it may be that the optical fibers 2 are fixed in a bent shape at the exposed portions 2a thereof (particularly, at a portion P shown) when the molding adhesive is poured between the end face of the columnar top portion 4c and the tip portions of the protective tubes 25, so that excess load is applied to the optical fiber 2 in some cases. Further, since the protective tubes may not be sufficiently fixed, excess load is exerted upon the optical fibers particularly at the portion P when the protective tubes are bent or sway.
d) In the optical fiber composite insulators, one or more optical fibers are passed through the through hole provided in the insulator body, the organic insulating material is filled in the through hole, and the organic insulating material is cured by heating. As is known, the curing temperatures of the organic insulating materials range from room temperature to beyond 100.degree. C. For example, Japanese Patent Application Laid-open No. 2-106,823 discloses that the curing temperature is set at not less than 60.degree. C. when the organic insulating materials is silicone rubber. Further, in order to cure the organic insulating material by heating, it is known that after the organic insulating material is filled into the insulator body at room temperature, the organic insulating material is cured by heating the entire insulator.
The organic insulating material filled in the through hole of the insulator expands or shrinks with changes in the surrounding temperature. At that time, the organic insulating material expands following the expansion on a temperature side higher than the curing temperature of the organic insulating material so that the optical fiber undergoes compression in a radial direction of the insulator. Therefore, when the insulator is heated to high temperatures through direct irradiation of sunlight in the summer or passage of current, the optical fiber is finely warped (microbending) due to expansion of the organic insulating material when the curing temperature is too low. Consequently, the light transmission loss increases. To the contrary, when the insulator is cooled to low temperatures with cold wind in the winter season or other reason and the curing temperature of organic insulating material is too high, the organic insulating material is shrunk to cause the optical fiber to be finely warped (microbending), so that the light transmission loss becomes greater, too.
Furthermore, when the entire insulator is heated after the organic insulating material is filled into the insulator body at room temperature, it takes a long time to heat the insulator to a given temperature because the heat capacity of the insulator is large. Consequently, the organic insulating material is cured at a temperature lower than the intended curing temperature, so that the light transmission loss becomes greater when the insulator is heated to high temperatures.
Furthermore, the optical fiber is finely bent (microbending) with a pressure exerted upon the fiber on filling the fluidizing organic insulating material by the shrinkage of the organic insulating material on curing, so that the sealing is effected in some cases in the state that the optical fiber is kept bent. If the optical fiber is sealed in the bent state, stress concentrates upon a bent portion of the optical fiber. As a result, the light transmission loss of the optical fiber increases, fatigue fracture is likely to occur due to expansion and shrinkage of the organic insulating material with changes in the surrounding temperature, and service life of the optical fiber decreases.